Assessment of the fetus during pregnancy, and particularly during labor and delivery, is an essential but yet elusive goal. While most patients will deliver a healthy child with or without monitoring, more than 5 out of every 1,000 deliveries of a viable fetus near term is stillborn, with half having an undetermined cause of death. (National Vital Statistics System (NVSS), CDC, NCHS as published in “Healthy People 2010, Understanding and Improving Health: Chapter 16,” co-authored by the Centers for Disease Control and Prevention and Health Resources and Services Administration, 2nd Edition, U.S. Government Printing Office, November 2000). The risk of this unfortunate consequence is increased in a subgroup of “high risk” patients (e.g. diabetics). In addition to regular obstetric observation, after 23 weeks gestation antepartum (“in utero”) fetal monitoring consists of the following (in order of complexity):    1. maternal report of fetal movement;    2. non-stress test (NST)—monitor fetal heart rate (FHR) by ultrasound, looking for baseline rate, variability and presence of accelerations above the baseline;    3. contraction stress test (CST)—response of the FHR to uterine contractions, either natural or induced; and    4. biophysical profile (BPP)—NST plus ultrasonographic evaluation of fetal movements and amniotic fluid volume.Despite their wide acceptance, these tests offer limited predictive value, and give only a glimpse of the fetus at the time of testing. For high risk patients, once or twice weekly surveillance is often indicated, entailing both expense and inconvenience for the patient.
Intrapartum fetal surveillance is accomplished routinely with intermittent auscultation or continuous Doppler monitoring of the FHR, together with palpation or tocodynamometry (strain gauge) monitoring of contractions. When indicated, more invasive monitors are available, but require ruptured membranes/adequate cervical dilation, and entail some risk, primarily infectious. These monitors include, without limitation:    1. fetal scalp electrode—a wire electrode inserted into the fetal scalp;    2. intra-uterine pressure catheter (IUPC)— enables quantitative measurement of contractions; and    3. fetal scalp sampling—a blood sample drawn for pH analysis.
Contraction detection allows monitoring of the progress of labor. The tocodynamometer detects only the presence or absence of tension on the abdomen (whether from uterine contraction or maternal movement), and often fails in the presence of obesity. When cervical dilation lags behind the anticipated labor curve, oxytocin is often indicated to induce a more effective contraction pattern. Safe titration of the oxytocin may require accurate determination of “Montevideo units” which measure the strength of uterine contractions over 10 minutes. This requires the more invasive IUPC, a catheter placed into the uterus, alongside the fetus, to measure the pressure generated by uterine contractions.
The rationale for use of intrapartum electronic fetal monitoring (EFM) assumes that FHR abnormalities accurately reflect hypoxia (inadequate oxygen to the fetus), and that early recognition of this could induce intervention to improve outcome for both mother and fetus. Unfortunately, numerous studies have failed to identify this improved outcome with the use of EFM in low-risk deliveries. In fact some studies have actually shown an increase in morbidity from a higher operative delivery rate. Perhaps this should not be surprising in light of the variability in interpretation of FHR tracings and their lack of specificity for hypoxia. Yet, continuous EFM remains the standard of care in US hospitals, in large part due to medicolegal concerns. Meanwhile researchers seek an alternative monitor, specific for fetal well being, preferably one that is non-invasive and comfortable for the mother, with reliable, reproducible interpretation. Recently, analysis of the fetal ECG (electrocardiogram) has held promise, with some features of the waveform more specifically indicating fetal hypoxia. Use of the waveform analysis reduced the incidence of severe metabolic acidosis at birth, while necessitating fewer scalp samples and operative deliveries. Unfortunately, acquisition of the FECG was through the fetal scalp electrode described above which is both invasive and limited in its application. The necessity for access to the fetal scalp requires both adequate cervical dilation and ruptured membranes, eliminating this procedure for antepartum fetal surveillance, as well as early labor.
Non-invasive acquisition of the FECG is a recognized issue of mixed signals. Electrodes placed on the skin surface will record all transmitted electrical activity including maternal ECG, maternal skeletal muscle, uterine muscle, fetal skeletal muscle, and fetal ECG.
Uterine contractions are the result of the coordinated actions of individual myometrial cells. At the cellular level, the contractions are triggered by a voltage signal called an action potential. During pregnancy, cellular electrical connectivity increases such that the action potential propagates to produce a coordinated contraction involving the entire uterus. The action potential during a uterine contraction can be measured with electrodes placed on the maternal abdomen resulting in a uterine EMG signal (hereinafter referred to as “EHG”: electrohysterogram). Specifically, the EHG signal can be processed to produce a signal that is similar to the standard uterine activity signal from the tocodynamometer or IUPC. The EHG provides contraction frequency and duration information. To date, EHG signals have not been used in assessing the intra-uterine pressure.
Postpartum, continuous uterine contraction is required to minimize uterine bleeding from the placental detachment site. Hemorrhage is the leading cause of peripartum maternal death, and most of these are postpartum hemorrhage due to this “uterine atony.” Current monitoring consists of serial uterine palpation at intervals of several hours. Diagnosis is usually made by patient complaint of severe bleeding, or hypovolemic shock (from hemorrhage). Neither IUPC nor tocodynamometer monitoring is available at this time. The EHG would provide a unique means for monitoring the uterine tone, providing an early warning of atony and potential hemorrhage.
Devices that utilize invasive techniques for monitoring fetal health include those disclosed in U.S. Pat. Nos. 6,594,515; 6,115,624; 6,058,321; 5,746,212; 5,184,619; 4,951,680; and 4,437,467.
To address the inadequacies noted above, various methods have been proposed for use in processing maternal abdominal signals to provide more accurate FECG extraction. These methods include subtractive filtering (see, for example, U.S. Pat. No. 4,945,917), adaptive filtering (see, for example, Widrow, B. et al., “Adaptive Noise Canceling: Principals and Applications,” Proc. IEEE, 63(12):1692-1716 (December 1975); Adam, D. and D. Shavit, “Complete Fetal ECG Morphology Recording by Synchronized Adaptive Filtration,” Med. & Biol. Eng. & Comput., 28:287-292 (July 1990); Ferrara, E. and B. Widrow, “Fetal Electrocardiogram Enhancement by Time Sequenced Adaptive Filtering,” IEEE Trans. Biomed. Eng., BME-29(6):458-460 (June 1982); U.S. Pat. Nos. 4,781,200 and 5,042,499), orthogonal basis (Longini, R. et al., “Near Orthogonal Basis Function: A Real Time Fetal ECG Technique,” IEEE Trans. On Biomedical Eng., BME-24(1):39-43 (January 1977); U.S. Pat. No. 5,042,499), linear combination (Bergveld, P. et al., “Real Time Fetal ECG Recording,” IEEE Trans. On Beiomedical Eng., BME 33(5):505-509 (May 1986)), single value decomposition (Callaerts, D. et al., “Comparison of SVD Methods to Extract the Fetal Electrocardiogram from Cutaneous Electrodes Signals,” Med. & Biol. Eng. & Comput., 28:217-224 (May 1990); U.S. Pat. No. 5,209,237), and MECG averaging and correlation (Abboud, S. et al., “Quantification of the Fetal Electrocardiogram Using Averaging Technique,” Comput. Biol. Med., 20:147-155 (February 1990); Cerutti, S. et al., “Variability Analysis of Fetal Heart Rate Signals as Obtained from Abdominal Electrocardiographic Recordings,” J. Perinat. Med., 14:445-452 (1986); J. Nagel, “Progresses in Fetal Monitoring by Improved Data Acquisition,” IEEE Eng. Med. & Biol. Mag., 9-13 (September 1984); Oostendorp, T. et al., “The Potential Distribution Generated by Fetal Heart at the Maternal Abdomen,” J. Perinat. Med., 14:435-444 (1986); U.S. Pat. No. 5,490,515). These methods, unfortunately, do not reliably enable continuous extraction of maternal-fetal data or cannot capture a comprehensive account of maternal-fetal health based on a combination of test results (i.e., fetal heart rate, fetal ECG, maternal ECG, and maternal uterine activity (EHG)).
Recently, magnetocardiography has been utilized in extracting fetal ECG (see, for example, Sturm, R. et al., “Multi-channel magnetocardiography for detecting beat morphology variations in fetal arrhythmias,” Prenat Diagn, 24(1):1-9 (January 2004); and Stinstra, J. et al, “Multicentre study of fetal cardiac time intervals using magnetocardiography,” BJOG, 109(11):1235-43 (November 2002)). Unfortunately, magnetocardiography is limited in application, technologically complex, and difficult to administer to assess accurate fetal ECG readings.
Accordingly, a cost-effective, portable monitoring system for both the mother and fetus is needed that can continuously monitor, in real-time, and accurately extract and evaluate maternal/fetal heart rates and ECGs, maternal EHG, as well as fetal position.